1. Field of the Invention
This invention relates to the manufacture of uniform, high quality, glass fiber mat products made by the wet-laid process, and, more particularly, it is concerned with a method of improving the wet-strength of freshly prepared, wet glass fiber mats, so that they can be conveniently handled and transferred, even mutually, in the production line.
2. Description of the Prior Art
High strength, uniform thin sheets or mats of glass fibers are finding increasing application in the building materials industry, as for example, in asphalt roofing shingles and as backing sheets for vinyl flooring. These glass fiber mats are replacing similar sheets made traditionally of asbestos fibers. Glass fiber mats usually are made commercially by a wet-laid process, which is carried out on modified paper or asbestos making machinery, as described, for example, in the book by O. A. Battista, Synthetic Fibers in Papermaking (Wiley) N.Y. 1964. A number of U.S. patents also provide a rather complete description of the wet-laid process, including U.S. Pat. Nos. 2,906,660; 3,012,929; 3,050,427; 3,103,461; 3,228,825; 3,760,458; 3,766,003; 3,838,995 and 3,905,067. The German OLS No. 2454354 (Fr. Demande No. 2,250,719), June, 1975, also is pertinent art in this field.
In general, the known wet-laid process for making glass fiber mats comprises first forming an aqueous suspension of short-length glass fibers under agitation in a mixing tank, then feeding the suspension through a moving screen on which the fibers enmesh themselves into a freshly prepared wet glass fiber mat, while the water is separated therefrom. However, unlike natural fibers, such as cellulose or asbestos, glass fibers do not disperse well in water. Actually, when glass fibers, which come as strands or bundles of parallel fibers, are put into water and stirred, they do not form a well-dispersed system. In fact, upon extended agitation, the fibers agglomerate as large clumps which are very difficult to redisperse.
In an attempt to overcome this inherent problem with glass fibers, it has been the practice in the industry to provide suspending aids for the glass fibers, including surfactants, in order to keep the fibers separated from one another in a relatively dispersed state. Such suspending aids usually are materials which increase the viscosity of the medium so that the fibers can suspend themselves in the medium. Some suspending aids actually are surfactants which function by reducing the surface attraction between the fibers. Unfortunately, however, none of the available suspending aids are entirely satisfactory for large volume manufacture of useful, uniform glass fiber mats.
In our copending patent application, Ser. No. 851,863, filed Nov. 15, 1977, there is described an improved method of preparing chopped glass fiber dispersions in water by admixing the fibers with a small amount of an amine oxide. While the present invention is not limited to this method of forming the initial glass fiber dispersions, it is to be considered a preferred embodiment thereof, and the examples which follow will reflect the advantageous use of amine oxide surfactants to form the initial glass fiber dispersions. However, any other dispersant may be used, including the select quaternary ammonium cationic surfactants described in our copending application, Ser. No. 876,651, filed Feb. 10, 1978 which compounds have the following formula: ##STR1## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are selected from the group consisting of aliphatic and aromatic hydrocarbon radicals, straight chain or branched, or two or more form a cyclic group, saturated or unsaturated, substituted or unsubstituted, at least two of said radicals containing at least 10 carbon atoms each, being the same or different, the sum of the carbon atoms in R.sub.1, R.sub.2, R.sub.3 and R.sub.4 being at least 22 and less than about 48, and X is an anion.
Of course, since the dispersing aid acts to suspend the individual glass fibers away from each other, the wet-strength of the wet mat formed on the screen may be lessened to some degree by the use of good dispersion aids, such as amine oxides.
The poorer wet-strength of such mats, however, does not mean poor strength of the dry mats and/or of the final dry and resin-bonded mat product, but it can create some problems during further processing of the wet mat. In commercial production of glass mats, for example, the wet mat formed on the foraminous belt is transferred to other units, such as the drying and the bonding resin application units of the production line. In each of these units, the wet mat is supported on felts or drums; however, it does remain unsupported at the transfer points from one unit to another. Furthermore, at the front end of the line, the wet mat from the foraminous belt often is manually transferred from one unit to another. If the wet mat as formed is too weak, it cannot be easily transferred manually from one unit to another. Furthermore, poor wet strength leads to occasional breakage of the mat in unsupported transfer areas during production, leading to undesirable interruptions and material waste.
The materials applied to the wet-mat in this invention to improve its wet-strength is to be distinguished from the conventional resin binders which are applied to the dry mat product. The former materials are used herein only to improve the wet strength of the wet mat sufficient to enable it to be transported, even manually, through the production line.
In general, for a wet-laid glass fiber process to be effective, it is necessary that it meet several rigid criteria simultaneously which can provide means for making the desired high quality, uniform finished glass fiber mat product at a rapid rate of production in an economically acceptable process. For example, the process preferably should provide a uniform dispersion of glass fibers in water effectively at low surfactant concentrations, at high glass fiber consistencies, preferably not be accompanied by a substantial increase in the viscosity of the medium, and should be capable of producing wet glass fiber mats at the screen which have a uniform distribution of fibers characterized by a multidirectional array of fibers.
The process also should provide means for improving the wet-strength properties of such freshly prepared wet glass fiber mats, so that it can be conveniently transferred, even manually, to other units in the production line, such as the drying and binding units, without tearing or breaking the wet mat during handling. The means for improvement in wet-strength of such wet mats should be effective for such mats formed from any suspending aid or dispensing surfactant, even with those which provide excellent glass fiber dispersions. The materials used for treating the wet mats to improve its strength preferably should be readily available, at low cost, and be capable of use either by direct spraying in dilute solution onto the wet mats at any convenient point in the production line.
These and other objects and features of the invention will be made apparent from the following more particular description of the invention.